He can hardly buy anything with it. We just give it value for the sake of giving it value.
You can hardly buy anything with gold, probably way less than with Bitcoin, yet it's still worth a lot. Bitcoin is a digital token that is secure in a way that no other digital asset is - Bitcoin does not rely on any third parties that must be trusted or can be ordered by government to freeze your money. Even if everyone who currently does stopped accepting Bitcoin as payment, it would still be worth a lot, because it's no longer solely a currency.